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Earthquakes and Volcanoes

Chapter 2 Lesson 2

Textbook pages 60-69

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*Earthquake

*Epicenter*Fault

*Focus

*Magma

*Seismic waves

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*Most of the time, Earth·splates move very slowly,

and we don·t feel themmove. Other times,Earth·s surface changessuddenly.

*Most major surfacechanges happen nearplate boundaries at whatare called faults.

*Faults are cracks inEarth·s crust where thereis movement.

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*What happens at the

faults?

|Rocks bend and fold

|Rocks lock together andjam

|Over many years, stressbuilds up on the rocks asthe plates push againsteach other until the rocksfinally break.

|The rocks suddenly´snapµ into a newposition, which causesthe Earth·s crust toshake

|There are many differentkinds of faults. (see p. 63)

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� Scientists use a toolcalled a seismograph the

waves of energy that areproduced during anearthquake.are calledseismic waves.

|When scientists talkabout how strong an

earthquake is (itsmagnitude), they use ascale called the Richterscale.

|We may not feel a 3.5

|7.5 is considered amajor earthquake

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|Focus: the point undergroundwhere the rocks are moving

|Epicenter: the point on thesurface directly above thesurface

|Waves that travel throughEarth·s interior (inside Earth)are called body waves

|P waves|S waves (travel slower)

|Waves that travel on thesurface are called L waves

|They are slower and do not goas far from the epicenter

 Also see p. 64 in the textbook

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*Most volcanoes start 37 to100 miles below Earth·ssurface where rock becomes

so hot that it melts.*This melted rock is called

magma.

*Melting rock releases gases,which mix with the magma,making it lighter than thesolid rock around it.

*Since it·s lighter, themagma rises. As it rises, itmelts the rock around it,forming a large chamber

below the Earth·s surface.

*

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*Near the surface, the

magma bursts through an

opening, called a vent.

Once it·s out, it is called

lava. The lava cools and

hardens on the surface.

*This cooled magma builds,

one layer at a time, to

form a volcano.

*Volcanoes also send out

rock fragments and gases

during an eruption.

*Uplifting

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*After an eruption, a volcano usually sinks in at the

top to form a crater.

*

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� The Pacific Plate is under the

Pacific Ocean.

� Around the edge of the Pacific

Ocean, earthquakes and

volcanoes are caused by thePacific Plate being forced

under other plates.

� What kind of boundaries do

you think these are?

� Converging

*